1. Field of the Invention
Embodiments of the invention generally provide a substrate support utilized in flat panel substrate processing.
2. Description of the Related Art
Liquid crystal displays or flat panel displays (FPD) are commonly used for active matrix displays such as computer and television monitors, personal digital assistants (PDAs), and cell phones, as well as solar cells and the like. Generally, a flat panel display comprises two glass plates having a layer of liquid crystal material sandwiched therebetween. At least one of the glass plates includes at least one conductive film disposed thereon that is coupled to a power supply. Power supplied to the conductive film from the power supply changes the orientation of the crystal material, creating a pattern such as texts or graphics on the flat panel displays. Generally, substrates utilized for flat panel fabrication are large in size, often exceeding 550 mm×650 mm, and are envisioned up to and beyond 4 square meters in surface area. Correspondingly, the substrate supports utilized to process large area substrates are proportionately large to accommodate the large surface area of the substrate.
Fabrication processes frequently employed to produce flat panel displays include chemical vapor deposition (CVD) and physical vapor deposition (PVD). Among them, plasma enhanced chemical vapor deposition (PECVD) for depositing thin film on a substrate is generally accomplished by introducing a precursor gas into a vacuum process chamber to be energized (e.g., excited) into a plasma. FIG. 1 is a schematic cross-sectional view of a process chamber 2 having a temperature controlled substrate support or susceptor 22 disposed therein to support a substrate. Reactive precursor gases, flowing into a gas manifold 16 through a gas inlet 14, a blocker plate 44 and a face plate 52 near the top of the process chamber 2, are excited to form a layer of material on the surface of the substrate. An opening 10 disposed in the sidewall allows a robot (not shown) to deliver and retrieve the substrate to and from the process chamber 2 by coordinating with a plurality of substrate support pins 24. The substrate support pins 24 are movably supported by a substrate pin plate 42 and are capable of passing through the susceptor 22 and moving up to receive the substrate to be delivered or retrieved by the robot. The susceptor 22, as supported by a shaft 20 and a lift mechanism, historically has been made of a single rectangular plate of aluminum and is typically heated by an embedded heater 32 with thermocouples and energy supplied from a power source 26.
Generally, the susceptor 22 of the process chamber 2 may be heated from room temperature to a high temperature of about 500° C. or less, and the susceptor 22 can defied and “droop” without adequate support. A substrate supported by the susceptor is prone to conform to the susceptor and, thus, also deflects. As a result, the vertical spacing between the gas manifold 16 and the substrate varies between central portions and the perimeter of the substrate, resulting in large degree of deflection or sagging and, thus, a greater distance near its perimeter. The difference in the vertical spacing (i.e., substrate deflection distance) greatly decreases uniformity of the deposited films disposed on the large area substrate.
Physical vapor deposition (PVD), or sputtering, is a plasma process performed in a vacuum process chamber where a negatively biased target with respect to a chamber body or a grounded sputter shield is exposed to a plasma of a gas mixture. Bombardment of the target by ions of the gas mixture results in ejection of atoms of the target material. The ejected atoms accumulate as a layer of deposited film on a substrate placed on a substrate support disposed within the PVD chamber.
A PVD process for flat panel fabrication generally operates at a lower temperature range which is about 200° C. lower than a CVD process. Thus, cooling, in addition to heating, of the substrate support for a PVD chamber is required. Especially after striking a plasma inside a PVD chamber, the energy from the plasma also creates heat directed to the substrate and the substrate support. Therefore, there is a problem of a temporal temperature increase or spike (e.g., about 30-50° C. increase from 150° C.) for the processing substrate disposed on the substrate support inside the PVD chamber. Such drastic temperature variation needs to be controlled in order to maintain a constant temperature on the substrate being processed. In addition, cooling of the substrate support of a PVD chamber is also needed after sputtering and during chamber part maintenance. However, for such a large area substrate, the performance of most cooling designs inside a PVD substrate support is not very good and there is a problem of too much local cooling which also leads to local temperature variations over a large area substrate. As a result, variations in film thickness, often manifesting as spots of thinner film thickness, have been observed, which is detrimental to the next generation of flat panel or solar cell devices.
Therefore, there is a need for an improved method and apparatus to control the temperature of a substrate support constantly to a desired range.